A core of a nuclear reactor is comprised of groups of a square or hexagonal cross section, each comprising a bundle of rods comprising the fissionable material. These groups are arranged side-by-side in the core of the reactor according to a network of square or triangular channels.
In order to bring about the best possible utilization of the nuclear fuel material and in order to avoid points of excessive flux, the arrangement of the groups is the object of predetermination by calculation, generally termed the loading plan of the core of the reactor.
During the first loading of the core of the reactor, the fresh groups, not yet irradiated, are distributed in regions according to the different ratios of enrichment in fissionable material which are specified in such a manner as to bring about a power distribution which is as uniform as possible.
After a cycle of irradiation, generally on the order of one year, each group has furnished a variable quantity of energy according to the initial ratio of enrichment of the fissionable material and according to the position occupied in the core of the reactor. It is necessary, therefore, to proceed with replacement of a certain number of groups which no longer possess but a weak potential for liberation of energy by an equal number of fresh groups. Furthermore, in order to make uniform the power distribution, new coordinates are assigned to each group, and different orientation in the core of the reactor is assigned as well. This new arrangement becomes the object of a new loading plan established by calculations, and determining for each group a new set of coordinates.
In order to bring about the disposition of the groups in conformance with the new loading plan, it is necessary to proceed according to a series of manipulations for removing the spent groups, replacing the fresh groups, and exchanging the rest of the groups in the core of the reactor.
Often, the reorganization of the groups in the core must be accompanied by an exchange of other components of the core, such as control rods, ion sources, clusters of plugs, of contaminants, etc.
All of these operations lead to a final state for which the core of the reactor contains a mixture of irradiated groups and fresh groups corresponding to a new loading plan with, for each group of the core, position specified by new coordinates. The modifications interposed in the core of the reactor then react together in the reactor pool where there is located a buffer rack, and in the deactivation pool (also known as a spent fuel pit) where is situated the stockpile or storage racks.
This set of manipulations must be carried out according to a rigorous procedure, termed a loading sequence.
The loading sequence comprises a series of instructions to be carried out in rigorous order, one after the other. According to the type and size of the reactor, a loading sequence comprises a number of variable instructions, sometimes greater than 500. These instructions are given to operators of the group manipulation machines. Each instruction carries an order number, the identification of each group or component involved in the manipulation, the localization of departure and the localization of destination. The loading sequence scrupulously prepared in advance and correctly executed, leads to an actual loading plan which will be identical to the specified loading plan.
It is important that the actual loading plan be verified in order to be forewarned against any loading error which could cause unacceptable hot spots and a poor utilization of nuclear fuel.
Actually, the verification is twofold. In the first phase of verification, coming about at the end of the loading sequence, each group which is immersed into the core of the reactor is verified by optical means in order to assure perfect correlation between the actual loading plan and the calculated loading plan. It is necessary notably to be assurer that the identification number engraved on the head of the group corresponds to that which is specified for the particular position in the specified loading plan. According to the subjective nature of this first verification, and the inherent difficulties in reading the identification numbers due to the state of fouling of the groups or due to lack of water clarity, a second verification phase is provided after replacement in the location in the reactor. When the interior of the reactor is re-closed, the connections are re-established and the reactor, returned to nominal conditions of pressure and temperature, has exceeded the critical state, there is effected a neutron flux distribution field in the core for comparison with calculated values, and then validation of the state of the core of the reactor before increasing the pressure. If this second phase of verification reveals an error, it is necessary to recommence entirely the loading and agree to a supplementary arrest for several days.